In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of radio frequency (RF) current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in a high-pressure gaseous lamp fill comprising the combination of a metal halide and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by RF current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a changing magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
Electrodeless HID lamps generally provide good color rendition and high efficacy in accordance with the standards of general purpose illumination. However, the lifetime of such lamps can be limited by erosion of the portion of the arc tube nearest the high intensity arc discharge. Erosion of the arc tube may be attributable to chemical reactions caused by intense ion bombardment and radiation from the arc discharge. For example, in an HID lamp containing a sodium iodide fill, as explained in EIectric Discharge Lamps by John F. Waymouth, M.I.T. Press, 1971, pp. 266-277, sodium iodide is dissociated by the arc discharge into positive sodium ions and negative iodine ions. The positive sodium ions are driven towards the arc tube wall by the electric field of the arc discharge. Sodium ions which do not recombine with iodine ions before reaching the wall may react chemically at the wall, or they may be lost by passing through the wall. Not only does the arc tube surface degrade, but as more and more sodium atoms are lost, light output decreases. Moreover, there is a buildup of free iodine within the arc tube that leads to arc instability and eventual arc extinction.